Bridge circuits have typically been employed to measure reactor values. (Hereinafter, the term "reactor value" will be taken to mean in a generic sense the amount of capacitance or the amount of inductance in a circuit element.) Bridges typically utilize in one of four arms a variable reactance that is adjusted until a null balance is achieved across the bridge. Two common measurement bridges are the Maxwell bridge either for comparing two inductances with each other or for comparing an inductance with a capacitance, and the Wien bridge for comparing two capacitances with each other. The balance equation for each of these bridges is independent of frequency.
The disadvantages of using these bridges, however, are that accurate results require a skilled operator and completion of a single measurement can require many minutes.
For measurement of small reactor values one can use a Q-meter employing a fixed frequency oscillator output applied to a series L-C circuit with either the capacitance or inductance as the unknown parameter. Current can be measured by a non-reactive hot wire ammeter. Knowing one reactor value and its resonant frequency when paralleled with the unknown reactance, the unknown reactor value can be computed. However, when a significant amount of resistance is present in the circuit, the total impedance at resonance may be only slightly different from that far from resonance and hence accurate measurement of the unknown value becomes difficult if not impossible.
Another reactor measuring device is taught by Procter et al. in their U.S. Pat. No. 3,970,925. Procter et al. utilize a variable frequency oscillator to produce a sinusoidal output which in turn is fed through a tank circuit which comprises reactive elements of known and unknown values. A phase detector measures the phase shift between the output voltage and current of the oscillator and supplies a phase shift signal specifying this information to a control circuit. The control circuit supplies a frequency control signal to adjust the frequency of the oscillator responsive to the phase shift signal in a direction tending to reduce the phase shift to zero or to some other predetermined value which indicates the resonance condition of the circuit.
It is necessary in the Procter et al. invention to compute the phase angle difference between the output voltage and current to determine the value of the unknown reactance element. Oftentimes a non-resonant phase must be utilized to determine if, for instance, a parallel circuit is employed (due to the fact that no current flows through the oscillator at resonance). This invention adjusts the frequency of oscillation through circuitry which comprises a phase detector and a control circuit which further comprises an amplifier and filter.
Accordingly, one of the objects of my invention is to provide a simple circuit for measuring the reactor value of an inductor or capacitor.
It is another object of my invention to provide a measuring device for a wide range of reactor values.
It is a further object of my invention to measure the quality factor of an unknown reactance element.